Papers by Author: Lukas Kwiatkowski

Abstract: Sheet-bulk metal forming is an innovative process with a high potential to generate load-adapted parts with high precision. Bulk forming processes of sheet metals especially require high process forces, resulting in an intense contact pressure and, thus, in a very high abrasive and adhesive wear. As a method to reduce or avoid these common wear phenomena, even hardened or coated tool surfaces are not sufficient. The objective of this paper is to show an improvement of the tool resistance during an incremental forming process by an adapted tool design and the application of structured tool surfaces combined with coatings. For the tool surface the structure of the scarabaeus beetle serves as the basis for a bionic structure. This structure was manufactured by micromilling. Despite the high hardness of the tool material and the complex geometry of the forming tools, very precise patterns were machined successfully using ball-end milling cutters. The combination of bionic structures with coating techniques like physical vapor deposition (PVD) on plasma nitrided tool surfaces is very promising. In this work, the influence of process parameters (workpiece material, lubrication, tool design, stepwise infeed) on the tool resistance during the forming operation was analyzed experimentally. The results of the optimized forming tools were compared to conventional, unstructured, uncoated, and only plasma nitrided forming tools. The different tools were applied to 2 mm thick metal sheets made of aluminum (AlMg3) and steel (non-alloy quality steel DC04). As a result, the process forces could be reduced by a modified shape and surface of the tools. Thus, the lifetime of the tools can be enhanced.

Abstract: The manufacturing of modern lightweight structures and the implementation of multi material concepts, for example in automotive engineering, entails appropriate joining technologies. The absence of additional connection elements or filling materials as well as the possibility to join dissimilar metals are basic requirements in this field of application to reach the aspired weight reduction. In case of tubular joints the die-less hydroforming process meets these demands and thus makes it an interesting alternative to conventional welding and riveting processes. The present work focuses on form fit joints produced by die-less hydroforming. It provides a verification of a previously presented analytical approach that allows the calculation of the working fluid pressure required to bulge the tube material into the groove of the outer joining partner. For that purpose, the groove filling characteristics of joined specimens with different groove geometries are analyzed. Here both joining partners were made of the aluminum alloy EN AW-6060. Additionally the connection strength of the joined specimens are determined using tensile tests. The results prove that the groove angle is the main influencing factor on the connection strength and that it can be used for an ordinal comparison of different groove geometries.

Abstract: In the field of sheet metal forming traditional forming processes are used. However, a quasi-static forming process combined with a high speed forming process can enhance the forming limits of a single one. In this paper, the investigation of the process chain quasi-static deep drawing – electromagnetic forming by means of a new coupled damage-viscoplasticity model for large deformations is performed. The finite strain constitutive model, used in the finite element simulation combines nonlinear kinematic and isotropic hardening and is derived in a thermodynamically consistent setting. The anisotropic viscoplastic model is based on the multiplicative decomposition of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong–Frederick kinematic hardening. Hill-type plastic anisotropy is modelled by expressing the yield surface as a function of second-order structure tensors as additional tensor-valued arguments. The coupling of damage and plasticity is carried out in a constitutive manner according to the effective stress concept. The constitutive equations of the material model are integrated in an explicit manner and implemented as a user material subroutine in the commercial finite element package of LS-Dyna with the electromagnetical modul. Aim of the work is to show the increasing formability of the sheet by combining quasi-static deep drawing processes with high speed electromagnetic forming process.

Abstract: The demand on closely-tolerated and complex functional components in the automotive sector, like e.g. synchronizer rings, leads to the development of a new process-class named “sheet-bulk metal forming”. Within this technology bulk metal forming operations are applied on sheet metals. In the following two novel approaches considering machines and tools for sheet-bulk metal forming are presented. The first approach aims on a technology based on rolling, which is suitable for mass production. The second one is an incremental forming solution for low batch production. Both machine concepts allow the application of different forming strategies to manufacture individual tailored semi-finished products in term of a pre-distribution of material. These products feature variable sheet thicknesses and mechanical properties, which can be adapted to their case of applica-tion. Depending on the individual batch size, the blanks can be finished to functional parts by sub-sequent forming processes like deep drawing and upsetting, extrusion or incremental forming. In this paper the case of an incremental tooth-forming is mainly considered. Forming sequences and resulting loads are modeled and calculated by finite elements simulations for all discussed processes to serve as a basis for the design and dimensioning of the machine components and forming tools.